Technologies are known for separating and recovering a target hydrocarbon gas (such as 1,3-butadiene) from a mixed gas containing hydrocarbons.
1,3-butadiene is an example of a hydrocarbon gas that is targeted for separation and recovery. 1,3-butadiene is a compound that is useful as a starting material for the production of synthetic rubber as well as an intermediate of an extremely large number of compounds. 1,3-butadiene is typically produced by thermal decomposition of naphtha or dehydrogenation of butene. In these production methods, 1,3-butadiene is obtained in the form of one component of a mixed gas. Thus, it is necessary to selectively separate and recover 1,3-butadiene from this mixed gas. Examples of main components in the mixed gas having four carbon atoms include 1,3-butadiene, isobutene, 1-butene, trans-2-butene, cis-2-butene, n-butane and isobutane. Since these compounds have the same number of carbon atoms and similar boiling points, they are difficult to separate from each other using industrial distillation methods.
Another example of a separation method is extractive distillation. Since this method is an absorption method that uses a polar solvent, such as DMF, an extremely large amount of energy is required when recovering 1,3-butadiene from the polar solvent. Thus, an adsorption method to separate and recover 1,3-butadiene using less energy is desired.
However, since conventional porous materials (Patent Document 1) exhibit low separation performance with respect to the target gas, multi-step separation is required, thereby leading to unavoidable increases in size of the separation apparatus.
Porous metal complexes that induce a dynamic structural change by an external stimulus have been developed as adsorbents that provide separation performance superior to that of conventional porous materials (Non-Patent Document 1 and Non-Patent Document 2). In the case of using the porous materials described in these publications as gas adsorbents, a unique phenomenon has been observed in which, although gas is not adsorbed below a certain pressure, gas begins to be adsorbed once that pressure is exceeded. In addition, a phenomenon has been observed in which the gate-opening pressure varies depending on the type of gas.
In the case of applying this porous material to an adsorbent in a pressure swing adsorption system, gas can be separated extremely efficiently. In addition, the range of pressure swing can be narrowed, thereby contributing to energy savings. Moreover, this can contribute to downsize and cost-reduction of the gas separation apparatus, enabling to enhance cost competitiveness for both high-purity gas products and finished products made from the high-purity gases.
However, to meet growing demands for even greater cost reductions, it is necessary to further improve adsorption and separation performance.
A metal complex [Zn(R-ip)(bpe)](wherein R represents H, Me, NO2 or I) has been disclosed and the complex is composed of a zinc ion, various types of isophthalic acid derivatives and 1,2-di(4-pyridyl)ethylene (Patent Document 2 and Non-Patent Document 3). However, although these disclosures contain discussions of the adsorption and separation characteristics relating to hydrocarbons having two carbon atoms, there is no discussion regarding the adsorption and separation characteristics of hydrocarbon gases having four carbon atoms, including 1,3-butadiene.
Attempts to change gate-opening pressure by mixing a plurality of ligands have been disclosed with respect to metal complexes consisting of a zinc ion, various types of isophthalic acid derivatives and 4,4′-bipyridyl (Patent Document 3 and Non-Patent Document 4). However, these disclosures do not discuss the 1,3-butadiene separation characteristics of metal complexes composed of bidentate organic ligands other than 4,4′-bipyridyl.